1. Field of the Invention
The present invention relates to an image reading apparatus and an image forming apparatus, which read a color document or a monochromatic document using a CCD sensor and form an image.
2. Description of the Related Art
In the prior art, a 3-line CCD sensor, which comprises three line sensors of red, green and blue, is generally employed for color image reading by an image reading apparatus. The 3-line CCD sensor is configured to have three one-dimensional line sensors that have light-receiving surfaces on which a red filter, a green filter and a blue filter are disposed, respectively. Because of this structure, it is not possible to read the same location on the original by all the line sensors at a time.
To cope with such a positional displacement in the original scan direction, image signals acquired by the respective line sensors are adjusted for alignment by using a memory circuit that is composed of, e.g. a line memory.
In recent years, 4-line CCD sensors have also been marketed as products. The 4-line CCD sensor is configured such that a line sensor for reading a monochromatic image is added to the 3-line CCD sensor for reading color images. The line sensor for reading a monochromatic image has a light-receiving surface on which no color filter is disposed.
As regards the CCD sensor comprising a plurality of line sensors, a system has begun to be developed as a product, which produces output signals of the respective line sensors by switching the output stage of the CCD sensor, thereby simplifying a rear-stage processing system.
At the output stage of the CCD sensor, the rear-stage processing system, etc. can be simplified by selectively using the outputs of the respective line sensors. However, signals with different amplitudes and waveforms are input to the circuit that processes analog signals in the rear stage of the CCD sensor. Consequently, such a problem arises that an image quality varies and a complex control is required.